The present invention is directed to a tubular part wall thickness measuring device and more specifically to a device utilizing a radiation source and detector having a length larger than the outside diameter of the tubular part being measured whereby parallel radiation beams emitted from the radiation source go through the detector at least through the entire section of the tubular part so that the average wall thickness of the tubular part can be determined from the amount of attenuation of radiation which is detected by the detector.
In general, in the manufacture of tubular pipes by conventional rolling operation in the steel industry the wall thickness of the tubular pipe must be measured with a high degree of accuracy. In order to increase productivity, it is essential to measure the wall thickness of the pipe on-line without stopping the flow of products. Furthermore, since the rolling operations generally involve a hot rolling step at very high temperatures, it is desireable that the wall thickness of the pipe be measured not only in a non-contact manner, but be measured at a distance as far as possible from the tubular part.
The arrangement of a conventional tubular part wall thickness measuring device is shown in FIG. 1 wherein .gamma.-ray sources 1, 2, and 3 emit radiation which is detected by radiation detecting units or sensors 4, 5, and 6. The .gamma.-ray sources 1 and 2 and the sensors 4 and 5 are mounted on a stationary frame 7 and the .gamma.-ray source 3 and sensor 6 are mounted on a moveable frame 8. The tubular part 11, whose wall thickness is to be measured, is conveyed along a conveyor 9 in a direction transverse to the direction of the .gamma.-rays.
In this operation the relative positions of the .gamma.-ray sources and the sensors are important factors. The moveable frame 8 should be positioned in FIG. 1 so that the vertices of the regular triangle EFG, as shown in FIG. 2, which is formed by the beams, are on the circumference of a circle whose diameter is the mean value of the nominal outside and inside diameters of the tubular part 11 (hereinafter referred to as "a middle diameter"). The principle of measurement will not be described in the present application, since it has been disclosed in the specification of Japanese Patent Application Laid Open No. 46406/1981 and is not essential for understanding of the present invention.
As the tubular part 11 moves along the conveyor 9, the tubular part 11 is vibrated in the direction of the axes Z.sub.1 -Z.sub.2 and Z.sub.3 -Z.sub.4 as shown in FIG. 2, at all times. Accordingly, even if a vibration preventing roller (not shown) is added to the conveyor rollers 9, it is extremely difficult to precisely set the vertices of the regular triangle EFG formed by the three beams on the circumference of the circle having the middle diameter. Additional means, such as a vibration preventing roller, include very technical and very costly problems. The conventional measuring device as shown in FIGS. 1 and 2 suffers from the drawback that its measurement theoretically includes an error due to vibration which is referred to as a "mis-alignment error". Accordingly, the utilization of a vibration preventing roller in conjunction with a conveying roller 9 to minimize the vibration of the tubular part 11 to minimize alignment error, has not been widely practised.
Another method of measuring the wall thickness of a steel pipe by means of radiation is disclosed in Japanese Patent Application Laid Open No. 114263/1979. In the conventional method, based on the fact that radiation applied to a steel pipe from outside is attenuated to the maximum when passed tangentially of the inner surface of the pipe and is attenuated to a minimum when passed tangentially of the outer surface of the pipe, each the maximum and minimum attentuation points are detected so that the wall thickness of the pipe can be determined from the distance between both.
However, when a steel pipe having a wall thickness of 5 or 6 mm to 40 mm is measured according to the foregoing method, even if the radiation source employs a radioactive material of 30 curies, it takes at least 20 ms to 1 second for measurement because the amount of radiation from the radioactive source is generally fractured. Therefore, during this period, the steel pipe must be held at rest. Accordingly, such a method cannot be used in measuring on-line the wall thickness of steel pipe which is vibrated while being conveyed. Furthermore, it can be understood that where the image of a radiation projected steel pipe is taken with a television camera with the width of a slit for projecting radiation from the radiation source set at about 2 mm, the steel pipe wall thickness measurement accuracy according to the method is much lower than that of steel plate thickness gauge, several tens of micrometers, because the resolution of the television camera is only about 1 mm.